Method for manufacturing an optical member formed of a fluoride crystal

ABSTRACT

A method for manufacturing an optical member of a fluoride crystal includes: a growing step of growing an ingot of a fluoride crystal; a cutting-out step of cutting out from the ingot a cylindrical basic material with two parallel planes which have a certain crystal plane orientation; an orientation-determining step of determining a crystal orientation of a side surface of the cylindrical basic material; a birefringence-measuring step of measuring birefringence in a specific crystal axis direction at the side surface determined based on the crystal orientation determined in the orientation-determining step; and an evaluating step of evaluating the fluoride crystal on the basis of a result of measurement of the birefringence. A fluoride crystal is obtained in which a maximum value of birefringence in a specific crystal axis direction at a side surface is not more than 10 nm/cm at a measurement wavelength of 633 nm. A high-resolution lens suitable for an oblique incident beam and an exposure apparatus using the lens are provided.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a manufacturing method formanufacturing an optical member for constituting an optical system in anoptical apparatus such as a camera, a microscope, or a telescope and anphotolithography apparatus such as a stepper as well as an opticalelement thereby obtained, and more particularly to an optical memberformed of a fluoride crystal used as an optical member forphotolithography of not more than 250 nm as well as a method formanufacturing the same.

[0003] 2. Description of the Related Art

[0004] In recent years, rapid development is being made in lithographytechnology for depicting integrated circuit patterns on wafers. Therehas been ever-increasing demand for higher integration of integratedcircuits, and in order to realize the higher integration it is necessaryto increase the resolution of a projection optical system of aprojection exposure apparatus. The resolution of a projection lens isgoverned by the wavelength of the light used and the numerical aperture(NA) of the projection lens. In order to increase the resolution, itsuffices if the wavelength of the light used is made shorter and NA ofthe projection lens is made larger (larger aperture).

[0005] First, a description will be given of the trend toward theshorter wavelength of the light. As for the wavelengths of light sourcesused in projection exposure apparatuses, the trend toward the shorterwavelength is underway from g-line (wavelength: 436 nm ) to i-line(wavelength: 365 nm ), and further from KrF excimer laser light(wavelength: 248 nm ) to ArF excimer laser light (wavelength: 193 nm ).If F₂ laser light (wavelength: 157 nm ) or the like whose wavelength isstill shorter is to be used in the future, it is no longer possible touse general optical glass as a lens material of an image-forming opticalsystem such as a projection optical system since a decline intransmittance presents a problem.

[0006] For this reason, as the optical system of an F₂ laser stepper, itis considered commonplace to use a fluoride crystal, e.g., calciumfluoride (fluorite), as the optical member.

[0007] Next, a description will be given of the trend toward the largeraperture. In order to satisfy the optical performance as an opticalmember for use in the optical system of the KrF, ArF excimer laserstepper or the F₂ laser stepper, it is considered the crystal materialis preferably a single crystal.

[0008] In addition, in conjunction with the trend toward the highperformance of the projection exposure apparatus, a large-diametercalcium fluoride single crystal having a diameter of φ100 mm to φ350 mmor thereabouts has recently come to be required. Because of its lowrefractive index and small dispersion (wavelength dependence of therefractive index) as compared with general optical glass, such a calciumfluoride (fluorite) single crystal is very effective in that chromaticaberrations can be corrected when used together with an optical memberformed of another material. In addition, as compared with the othercrystal materials (barium fluoride), the calcium fluoride (fluorite)single crystal is easily available from the market, and large-diametersingle crystals with diameters of φ100 mm or more are available.

[0009] The calcium fluoride single crystals having these advantages haveconventionally been used as a lens material for cameras, microscopes,and telescopes in addition to an optical material for steppers. Inaddition, single crystals of barium fluoride and strontium fluoride,which are fluoride single crystals other than the calcium fluoridesingle crystal, have recently attracted attention as a next-generationoptical material in view of the fact that these single crystals belongto the same isometric system and their properties are similar.

[0010] As for the fluoride single crystals, single-crystal growingmethods are known including melting methods such as the Bridgman method(Stockbarger or lowering method) and the Tammann method.

[0011] One example of the method of manufacturing a calcium fluoridesingle crystal in accordance with the Bridgman method is shown below.

[0012] In the case of the calcium fluoride single crystal used inultraviolet to vacuum ultraviolet regions, natural fluorite is not usedas the material, and a high-purity material manufactured by chemicalsynthesis is generally used.

[0013] The material can be used in powder form, in which case thereduction in volume is intense when it is melted, so that a semi-moltenproduct or a pulverized product thereof is generally used. First, acrucible with the material filled therein is placed in a growingapparatus, and the interior of the growing apparatus is kept in a vacuumatmosphere of 10⁻³ to 10⁻⁴ Pa. Then, the temperature within the growingapparatus is increased to a level above the melting point of calciumfluoride (1370° C. to 1450 ° C.) to melt the material. At this time,control based on constant power output or high-precision PID control iseffected so as to suppress the temporal change of the temperature withinthe growing apparatus.

[0014] At the stage of crystal growth, the crucible is lowered at aspeed of 0.1 to 5 mm/h or thereabouts, thereby allowing the material togradually crystallize starting with a lower portion in the crucible.When crystallization has occurred up to an uppermost portion of themelt, crystal growth is completed, and simple annealing is effected byavoiding quenching so that the grown crystal (ingot) will not fracture.When the temperature within the growing apparatus has lowered to roomtemperature or thereabouts, the apparatus is opened to atmosphere, andthe ingot is removed.

[0015] In this crystal growth, a graphite-made crucible is normallyused. As for its shape, the graphite-made crucible is of a pencil typewhose tip portion is conical and whose remaining portion is cylindrical.By allowing the crystal to grow from the pencil-type tip located at alower end of the crucible, a crystallized ingot is obtained. Inaddition, it is practiced to insert a seed crystal in the tip portion tocontrol the orientation of the crystal plane of the ingot to somedegree, but if the diameter of the ingot exceeds φ100 mm, control of theorientation becomes extremely difficult.

[0016] It is considered that the fluoride crystal fabricated by theBridgman method basically has no predominance in the orientation ofgrowth, and the horizontal plane of the ingot becomes a random plane foreach crystal growth.

[0017] After crystal growth, since a large residual stress is present inthe removed ingot, simple heat treatment is effected in the shape of theingot as it is.

[0018] The ingot of the calcium fluoride single crystal thus obtained iscut and processed to an appropriate size by target product type. Here,in a case where the orientation of the crystal plane is not taken intoconsideration, to efficiently cut out a larger basic material formanufacturing an optical element (lens or the like) from the ingot, theingot is cut (sectionally cut) horizontally. Subsequently, the basicmaterial cut out is subjected to heat treatment for obtaining desiredoptical performance (homogeneity of the refractive index andbirefringence).

[0019] As one example of the method for manufacturing a calcium fluoridesingle crystal using the above-described Bridgman method and heattreatment, Japanese Patent Application Laid-open No. 8-5801, forexample, filed by the assignee of the present application discloses amethod for obtaining an optical material whose refractive indexdifference is 5×10⁻⁶ or less by providing heat treatment after growing afluorite single crystal by the Bridgman method.

[0020] Incidentally, since the fluoride single crystal exhibits highoptical performance in a direction normal to a {111} crystal plane ascompared to other crystal planes, it is practiced to measure the {111}crystal planes of an ingot of the fluoride single crystal, and to cutout a basic material for manufacturing an optical element such that the{111} planes become two parallel planes, followed by heat treatment.

[0021] Alternatively, a method is practiced in which after the ingot ofthe fluoride single crystal obtained by crystal growing is subjected toheat treatment for obtaining desired optical performance (homogeneity ofthe refractive index and the birefringence), and a basic material forfabricating an optical element is cut out so that the {111} crystalplanes become two parallel planes, so as to obtain a fluoride singlecrystal of high optical performance.

[0022] Meanwhile, since the intrinsic birefringence of the fluoridecrystal becomes greater as the measurement wavelength becomes shorter,in a projection lens of 193 nm or less, studies are being made to use afluoride crystal in which the {100} planes or the {110} planes are twoparallel planes, in addition to the fluoride crystal of the {111} plane.Even in the case of the fluoride crystal in which the {100} planes orthe {110} planes are two parallel planes, a cutting-out step and a heattreatment step similar to those of the {111} plane are carried out.

[0023] In addition, to manage the intrinsic birefringence of thefluoride crystal and alleviate its effect, measurement of the crystalplane orientation in at least two directions by, for instance, the Lauemethod has been practiced.

[0024] In this document, the birefringence is a phenomenon in which therefractive index differs depending on the polarizing direction of thelight (i.e., electromagnetic waves), and the polarizing direction inwhich the refractive index becomes minimum is referred to as the “fastaxis,” and the polarizing direction in which the refractive indexbecomes maximum is referred to as the “slow axis.” The birefringence isgenerally represented by the optical path difference (calledretardation) between the polarized light on the fast axis and thepolarized light on the slow axis at the time when the light passes aunit length of a substance, and nm/cm is used as the unit. In addition,the birefringence occurs not only due to the intrinsic birefringenceinherent in the substance or the crystal structure, but also there arecases where the birefringence occurs due to strains attributable tothermal stress and the like. There are cases where such a birefringenceis simply called a strain.

[0025] The intrinsic birefringence has a value inherent in the substanceindependently of the crystal fabrication method and heat treatmentconditions. Accordingly, even if the amount of birefringence and theorientation of the fast axis are not measured, if only the crystal planeorientation is measured, the amount of birefringence can be managed, andit is possible to overcome its effect by combining a plurality ofoptical members.

[0026] Meanwhile, with respect to the occurrence of the birefringenceattributable to the thermal stress, by providing heat treatment in thesame way as multicomponent optical glass and silica glass, the value ofbirefringence after heat treatment of the fluoride crystal becomes 1 to2 nm/cm or thereabouts (measurement wavelength: 633 nm ) in themeasurement in the optical axis direction. Thus it has been thought thatthe stress attributable to the thermal stress is lowered to a levelwhich does not hamper free optical design.

[0027] For example, Japanese Patent Application Laid-open NO. 11-240798filed by the assignee of the present application discloses a method formanufacturing a calcium fluoride single crystal having a large diameterand a small birefringence which is usable for photolithography with awavelength of 250 nm or less. This method is characterized by subjectingthe calcium fluoride single crystal to heat treatment under a specifictemperature schedule. It is reported that, in this method,birefringences in a lateral direction perpendicular to the optical axisof the calcium fluoride single crystal after heat treatment weresubstantially the same at the rotational angle of 360° ([0047] ofJapanese Patent Application Laid-open NO. 11-24079). For this reason, inits embodiment, the birefringence in a lateral direction (at anarbitrary direction) was measured without determining the crystalorientation in the lateral direction.

[0028] As a result of measuring the amounts of birefringence in variousplane orientations of the calcium fluoride single crystal, the presentinventors found that birefringences which cannot be suppressed by heattreatment remain in the calcium fluoride single crystal. Further, it wasfound that such a birefringence is at a level imparting adverse effectson the optical design. Furthermore, it was found that when a pluralityof optical members are combined, this type of birefringence is not ableto offset the effects. In other words, it is necessary to fabricate anoptical system with optical members in which this type of birefringenceis suppressed to a certain level or less.

SUMMARY OF THE INVENTION

[0029] The present invention has been devised in view of theabove-described problems, and it is an object of the present inventionto provide an optical member formed of a fluoride single crystal inwhich the effect of birefringence is minimized, as well as a method formanufacturing the same. Another object of the present invention is toprovide an exposure apparatus having the optical member of the presentinvention.

[0030] In accordance with the present invention, there is provided amethod for manufacturing an optical member of a fluoride crystal,comprising:

[0031] a growing step of growing an ingot of a fluoride crystal;

[0032] a cutting-out step of cutting out from the ingot a cylindricalbasic material with two parallel planes which have a certain crystalplane orientation;

[0033] an orientation-determining step of determining a crystalorientation of a side surface of the cylindrical basic material;

[0034] a birefringence-measuring step of measuring birefringence in aspecific crystal axis direction at the side surface determined based onthe crystal orientation determined in the orientation-determining step;and

[0035] an evaluating step of evaluating the fluoride crystal on thebasis of a result of measurement of the birefringence.

[0036] When the present inventors measured the amounts of birefringencein various plane orientations of the calcium fluoride single crystal, itwas found that there are cases where the birefringence attributable tothermal stress cannot be sufficiently suppressed according to aconventional method. Then, when a further study was made, it was foundthat the amount of birefringence at a side surface perpendicular to theoptical axis of the calcium fluoride single crystal occurring due to thethermal stress varies substantially in dependence of the crystalorientation. Accordingly, the present inventors succeeded in sorting anoptical member having excellent optical characteristics by determiningin advance a specific crystal plane orientation in which thebirefringence attributable to thermal stress becomes large, and bymanaging the amount of birefringence in that orientation. For example,in the case where two parallel planes are the {111} planes, it sufficesif a determination is made as to whether or not the amount ofbirefringence in a specific crystal direction <110> at a side surface isa prescribed value or less. In the case where two parallel planes arethe {100} plane, respectively, it suffices if a determination is made asto whether or not the amount of birefringence in a specific crystaldirection <110> or <100> at the side surface is a prescribed value orless. Then, it suffices if only the member of the prescribed value orless is used as a material for forming an optical system such as a lens.

[0037] In particular, since optical lenses with large NA have recentlybeen used to improve the resolution, in the light passing through thelens, components oblique to the optical axis increase in the light.These oblique components are affected by the distribution of therefractive index not only in the optical axis direction but in adirection perpendicular to the optical axis, i.e., in a lateraldirection of the cylindrical crystal (in-plane direction perpendicularto the optical axis). Accordingly, by inspecting the amount ofbirefringence in the lateral direction perpendicular to the optical axisin accordance with the present invention, it becomes possible todetermine whether or not the optical member is suitable for an opticalsystem used for an exposure apparatus or the like.

[0038] In the above-described orientation-determining step, the crystalorientation at a side surface is determined by measuring thebirefringence at the side surface at a plurality of angles. In theevaluating step, an evaluation can be made as to whether or not amaximum value of the birefringence in the specific crystal axisdirection at the side surface is 10 nm/cm or less at a measurementwavelength of 633 nm. In a projection optical system used for anexposure apparatus, excellent image-forming characteristics are obtainedby suppressing the maximum value of the birefringence in the specificcrystal axis direction at the side surface to 10 nm/cm or less at themeasurement wavelength of 633 nm. In the case where the maximum value ofthe birefringence in the specific crystal axis direction at the sidesurface is 10 nm/cm or less at the measurement wavelength of 633 nm, theaforementioned basic material can be formed into the shape of apredetermined optical member. If the case is otherwise, the cylindricalbasic material can be appropriately disposed of as being unsuitable forthe material of the optical member.

[0039] According to the present invention, an optical member of afluoride crystal manufactured by the manufacturing method of theinvention is provided. In this optical member, a maximum value of thebirefringence in a specific crystal axis direction at a side surface ofthe fluoride crystal, which is shaped in a cylindrical shape with twoparallel lined having a specific crystal plane orientation, is not morethan 10 nm/cm at a measurement wavelength of 633 nm. The aforementionedfluoride crystal may be a calcium fluoride single crystal.

[0040] According to the present invention, an exposure apparatus havingthe optical member manufactured by the manufacturing method of thepresent invention is provided. This exposure apparatus has an excimerlaser or an F₂ laser as a light source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 is a conceptual view illustrating one example of a methodfor manufacturing an optical member in accordance with the invention;

[0042]FIG. 2 is a conceptual view illustrating a method of growing acalcium fluoride single crystal;

[0043]FIG. 3 is a view illustrating an apparatus for measuring a crystalplane orientation in accordance with the Laue method (side reflectionmethod);

[0044]FIG. 4 is a conceptual view of α and β in a cylindrical member;

[0045]FIG. 5 is a conceptual view illustrating one example of aprojection exposure apparatus;

[0046]FIG. 6 is a schematic view illustrating an example of a projectionoptical system;

[0047]FIG. 7 is a schematic view illustrating another example of theprojection optical system;

[0048]FIG. 8 is a schematic view illustrating still another example ofthe projection optical system; and

[0049]FIG. 9 is a schematic view illustrating a further example of theprojection optical system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050]FIG. 1 shows an example of the flow chart of the method formanufacturing an optical member in accordance with the invention. Themethod for manufacturing an optical member in accordance with theinvention basically includes the step of growing a crystal; the step ofcutting out a cylindrical basic material and measuring the crystalorientation of a side surface of the basic material and birefringence;and a sorting step for determining the quality of the fluoride crystalor a method of use in accordance with the result of that measurement.

[0051] A crystal growing step 601 specifically includes a raw-materialrefining step, a preprocessing step, a crystal growth step, an annealingstep in a crystal growing furnace, and the like.

[0052] A measurement step 602 for measuring the crystal planeorientation in ingot form is sometimes carried out prior to the step ofcutting out the cylindrical basic material.

[0053] A cutting-out step 604 for cutting out the cylindrical basicmaterial specifically includes an inspection step for inspecting foreignobjects and the like in the inside, a cutting step, a rounding step, aplane polishing step, a chamfering step, and the like. In addition, atthe time of cutting, a step 603 for determining a cutting plane and acutting position by utilizing the Laue method, which will be describedlater, and a cleavage plane is taken.

[0054] After the cutting-out step, a step for measuring the crystalorientation of a side surface of the basic material and birefringence istaken, as required.

[0055] Further, an annealing step (heat treatment step) 605 is effectedwith respect to the cut-out basic material. The annealing stepspecifically has a temperature raising step for gradually raising thetemperature within an annealing furnace, a holding step for holding thebasic material at a fixed temperature, a temperature lowering step forgradually lowering the temperature, and a cooling-down step for allowingthe basic material to cool by turning off a heater of the annealingfurnace.

[0056] After the annealing step, a step 606 for measuring thebirefringence of the basic material after annealing is executed.Specifically, measurement is made of the amount of in-planebirefringence of two parallel planes of the basic material, thedistribution of the orientation of the fast axis, the amount ofbirefringence in the lateral direction, the orientation of the fastaxis, and the like. The sorting of the basic material is effected on thebasis of the results of this measurement. If the desired amount ofbirefringence has been obtained, the basic material is processed as itis into the lens shape (step 607), and undergoes an assembling step 608to form an optical system. If the desired amount of birefringence hasnot been obtained, the basic material is subjected again to, forinstance, the annealing step, and is thereby processed to assume adesired amount of birefringence.

[0057] The manufacturing method in accordance with the invention ischaracterized in measuring the crystal orientation in the lateraldirection of the cylindrical basic material having two parallel planescut out according to a predetermined crystal plane orientation, as wellas the amount of birefringence in that orientation, the orientation ofthe fast axis, and the like.

[0058] The crystal plane orientation of the basic material does notchange by heat treatment, but as for birefringence, the amount ofbirefringence and the orientation of the fast axis change before andafter the heat treatment. Accordingly, in the present invention, it ismade essential that the crystal plane orientation is measured at atleast any one of the stages of the ingot state, before heat treatment,and after heat treatment, and that birefringence (amount and the fastaxis) is measured at least after heat treatment.

[0059] For optimization of the birefringence, it is preferable tomeasure the crystal plane orientation and birefringences at therespective orientations before heat treatment, and appropriatelydetermine the heat treatment conditions on the basis of thisinformation.

[0060] It should be noted that when the crystal plane orientation in thelateral direction is measured before heat treatment, that orientationcan be marked on the basic material, and that the birefringence in theorientation of that marking can be measured after heat treatment.

[0061] Further, it was confirmed by the present inventors that theamount of birefringence in the lateral direction has periodicity in thecircumferential direction with respect to the crystal plane orientation,as will be described later. Accordingly, it is also possible to estimatethe crystal plane orientation by measuring the amount of birefringencein the lateral direction from a plurality of angles without measuringthe crystal plane orientation in the lateral direction. Namely, thecrystal orientation of the side surface can be determined by measuringthe amount of birefringence at a plurality of angles in the lateraldirection.

[0062] Hereafter, a detailed description will be given of the respectivesteps.

[0063] <Crystal Growing Step>

[0064] As for the crystal growing step, a method similar to aconventional method for manufacturing a fluoride crystal is used.Hereafter, a description will be given of the method of growing acalcium fluoride single crystal according to the Bridgman method(Stockbarger or lowering method).

[0065]FIG. 2 is a conceptual view illustrating the method of growing acalcium fluoride single crystal.

[0066] A high-purity material fabricated by chemical synthesis is usedas a raw material. First, a semi-molten article of the material isfabricated by the following procedure. A graphite crucible with thepowdered material filled therein is placed in a superposed manner insidea preprocessing apparatus, and the interior of the preprocessingapparatus is kept in a vacuum atmosphere of 10⁻³ to 10⁻⁴ Pa. Next, thetemperature within the preprocessing apparatus is raised to a levelabove the melting point of calcium fluoride (1370° C. to 1450° C.) tomelt the raw material, and the temperature is subsequently lowered toroom temperature. In this case, PID control is preferably effected so asto suppress the temporal change of the temperature within the apparatus.A fluorinating agent such as lead fluoride is added to this powderedmaterial. A semi-molten article thus obtained is moved to a crystalgrowing furnace, and after the temperature is raised again to themelting temperature, the crucible is lowered at a rate of 0.1 to 5 mm/hor thereabouts in the crystal growing stage, thereby allowingcrystallization to gradually take place starting from a lower portion ofthe crucible. When crystallization has occurred up to an uppermostportion of the melt, crystal growth is completed, and annealing (slowcooling) is effected by avoiding quenching (rapid cooling) so that thegrown crystal (ingot) will not fracture. When the temperature within thegrowing apparatus has lowered to room temperature or thereabouts, theapparatus is opened to atmosphere, and the ingot is removed.

[0067] In this crystal growth, by using a φ300 mm-diameter crucible 702made of graphite (carbon), a pencil-type ingot whose tip portion isconical is fabricated in a growing furnace 701 having heat insulatingproperties and airtightness. In this case, a single crystal can beformed by growing a crystal starting with a tip of the conical portionlocated at a lower end of the crucible. The crucible 702 is verticallymovable by means of a support 703. In addition, a high-temperature sideheater 704 and a low-temperature side heater 705 are disposed along theinner surface of the growing furnace 701 in an upper portion and a lowerportion, respectively, of the growing furnace 701, so that the growingfurnace 701 can be heated so that the lower portion of the furnaceinterior becomes lower than the upper portion thereof. Further, anexhaust line 706 for reducing the pressure within the furnace isinstalled in the lower portion of the growing furnace 701. First, a seedcrystal is placed in a tip portion of the crucible. This seed crystal isaimed at controlling the orientation of crystal growth, but since thehorizontal plane of the ingot actually becomes a random plane for eachcrystal growth, the orientation of the plane cannot be estimated at thisstage.

[0068] In the growing step, as shown in FIG. 2A, the crucible with thematerial of the fluoride single crystal filled therein is first placedin an upper portion of the growing furnace 701, and air is ventedthrough the exhaust line to keep the interior of the growing furnace ina vacuum atmosphere of 10⁻³ to 10⁻⁴ Pa. Next, the temperature within thegrowing furnace is raised to a level above the melting point of thefluoride (in the case of calcium fluoride: 1370° C. to 1450° C.) to meltthe raw material.

[0069] Then, as shown in FIG. 2B, the crucible 702 is lowered at apredetermined rate (0.1 to 5 mm/h) by means of the support 703, and amelt 707 is allowed to gradually crystallize starting with the lowerside of the crucible, thereby growing a fluoride single crystal 708.

[0070] Next, as shown in FIG. 2C, when the melt 707 has crystallized inthe crucible up to an uppermost portion thereof, crystal growth iscompleted. The temperature is allowed to drop gradually inside thefurnace down to room temperature or thereabouts so as to prevent a growncrystal 709 (ingot) from fracturing and to reduce the residual stress,and the ingot is subsequently taken out (A).

[0071] <Plane Orientation Measuring Step>

[0072] When a cylindrical (columnar) basic material is cut out from theingot, the crystal orientation of at least two parallel planes isdetermined. Further, in this stage, two or more crystal orientations maybe measured by using the Laue method or the like. Furthermore, theinvention is not limited to measuring the crystal orientation of theentire ingot, and a test piece may be cut out from a top portion or acone portion of the ingot and its crystal orientation may be measured,thereby determining the crystal orientation of the entire ingot. Inaddition, the crystal orientation may be measured in the state of amatrix in which the ingot has been cut or processed to some extent.

[0073] In the measurement of the plane orientation, the Laue method, forexample, is used in which the crystal plane orientation is measured byirradiating a sample with X-rays.

[0074] The Laue method has an advantage in that various crystal planeorientations of such as {111}, {110}, and the like can be simplymeasured and controlled.

[0075] Since the Laue method is based on a lateral reflection method,the Laue method has an advantage in that measurement of even alarge-diameter sample is possible without imparting damage to thesample.

[0076] At the time of determining the crystal orientation, it isdesirable to suppress an angle of deviation from a desired orientationto within 3°. A detailed description will be given of a method ofmeasuring the crystal orientation which is suitable for the invention.

[0077] Methods of evaluating the crystal orientation include methodsbased on X-rays, mechanical methods, optical methods, and the like.

[0078] Methods of evaluating the crystal orientation by X-ray includethe Laue method in which the crystal set stationary is irradiated withX-rays, a rotating method or a vibrating method in which the crystal,while being rotated or vibrated, is irradiated with X-rays, theWalsenburg method or the precession method in which these methods aremodified, and so forth.

[0079] Next, a description will be given of the mechanical methods.

[0080] If a plastic deformation is imparted to a crystal by anappropriate means, various surface patterns which are characterized bythe crystal orientation appear on its surface. For example, such surfacepatterns include pressure figures (or percussion figures) havinginherent shapes in the crystal planes, as well as slip bands, twins, andcleavages occurring along specific crystal planes. Of these, twincrystals include, in addition to plastic deformation by twinning,annealed twins and grown twins, and they similarly produce surfacepatterns.

[0081] Specifically, the mechanical methods include a method making useof a pressure figure, a method making use of a slip ellipse, a methodmaking use of an angle of intersection between slip lines, twins, andother surface patterns, a method making use of a cleavage plane,analysis of slips, twins, and cleavages, and so forth.

[0082] In addition, the optical methods include the goniometric method,the etching-figure method, the light figure method, the ellipsometricmethod, and the like.

[0083] Of these measuring methods, the method using X-rays yields highmeasurement accuracy and a high speed, this method is suitable for usein the present invention. Hereafter, a description will be given of themethod using X-rays.

[0084] In a case where an X-ray diffractometer is used, a Laue camerafor back surface reflection is installed on a side opposite to adiffractometer of an X-ray tube. The distance between the sample surfaceand the film is set to several dozen millimeters. The X-ray tube uses aMo target, and filming is effected at a tube voltage of 40 kV and a tubecurrent of 50 mA for an exposure time of 60 sec. The analysis of theorientation is effected by hand calculation from a polaroid photographof the Laue pattern obtained, or calculated by fetching the photographinto a computer by a scanner.

[0085] The Laue method is one of X-ray diffraction methods, and is soarranged that white X-rays strike a fixed single crystal. Since theBragg angle θ is fixed with respect to all the planes of the crystal,each plane undergoes diffraction by selecting X-rays of a wavelength λwhich satisfies the Bragg condition λ=2dsinθ with respect to the Braggangle θ. As for the Laue method, there are three methods including thetransmission method, the back-reflection method, and the side-reflectionmethod by varying the relative positional relationship of the X-raysource, the crystal, the film or CCD camera. In the transmission method,the film or CCD camera is placed in the rear of the crystal to recordbeams which are diffracted in the forward direction. In theback-reflection method, the film is placed midway between the crystaland the X-ray source, and incident beams are passed through an apertureformed in the film, and the beams that are diffracted in the backwarddirection are recorded. In the side-reflection method, the X-ray sourceis disposed such that beams are made incident upon the crystal at acertain incident angle {overscore (ω)}, and the film or CCD camera isplaced at a position rotated by φ with respect to the incident beam torecord detracted beams in arbitrary side directions. In each of themethods, diffracted beams form Laue spots on the film or a fluorescentscreen. Since the position of the Laue spot is determined by therelative relation of the crystal orientation with respect to theincident beam in each method, the position of the Laue spots is used inthe determination of the crystal orientation by applying this fact.

[0086] The Laue method makes it possible to easily measure variouscrystal plane orientations of such as {111} and {110}, and is suitablefor the present invention in terms of both measurement accuracy and thespeed.

[0087] It should be noted that as a method of indicating the crystalorientation, Miller index is used. The Miller index is the reciprocal ofa ratio of the distance from the origon of a unit lattice of a crystalto the point of intersection between the plane and the crystal axis tothe unit length of the axis. In the case of a cubic system such ascalcium fluoride, if it is assumed that the unit length of each crystalaxis is a, the Miller index is (hkl) in a case where a certain planeintersects the axis at points of a/h, a/k, and a/l. In the cubic system,the orientation [hkl] is always perpendicular to the plane (hkl) of thesame index, and directions which are in a symmetrical relation arerepresented by one index, and are shown by being bracketed by < >.Meanwhile, equivalent lattice planes which are in a symmetrical relationare also represented by one index, and are shown by being bracketed by {}. For example, oblique lines of a cube, such as [111], [1-11], [-1-11],and [-111] are all represented by <111>, and surfaces of a cube, (100),(010), (-100), (0-10), (001), (00-1) are represented by {100}.

[0088] An apparatus for measuring the crystal orientation by the Lauemethod consists of an X-ray source, a sample stage, and a CCD camera(FIG. 3). X-rays are made incident upon the sample, and diffracted beamsthereby obtained are analyzed as a Laue pattern.

[0089] In the present invention, it is proposed that the followingmethod be preferably used for the measurement of a large sample such asa φ300×t60 block.

[0090] First, a measurement sample 800 is flatly placed on a stage 810,and an X-ray source (X-ray tube) 820 and an optical system of a film orCCD camera 830 are installed therebelow. Since the measurement sample800 is flatly placed on the stage 810, it becomes possible to cope witha large sample. In addition, detailed measurement of mapping and thelike is also made possible, and it suffices if spots are selected and adetermination is made as to whether or not the simulation is correct,thereby enabling efficient measurement. Furthermore, as a result of theside-reflection method, damage to the sample by X-rays is made varysmall.

[0091] Measurement of the crystal plane orientation by such a Lauemethod is executed. It should be noted that the measurement of thecrystal plane orientation is synonymous with the measurement of thecrystal axis. Namely, the measurement of the {111} plane orientation isequivalent to the measurement of the <111> axis.

[0092] By using the above-described measurement method, the planeorientation is managed in the procedure of cutting the top and the coneof the ingot, orientation measurement, estimation of the orientation ofthe ingot from data on the top and the cone, cutting out of the member,and measurement of the outside of the effective diameter of the member(mapping depending on the case) in the order mentioned.

[0093] Hereafter, the procedure of determining the plane orientation ofthe fluoride single crystal obtained by the Bridgman method will bespecifically shown.

[0094] As for the ingot of the fluoride single crystal, a side portionwhich was facing the front in the furnace is scraped with a wire brushand is made smooth, and one straight line is drawn with a glass penciland is set as a reference line of the position (B).

[0095] Subsequently, the conical shape (which will be referred to as acone portion) at the tip and its opposite portion (which will bereferred to as a top portion) are cut to a thickness of 30 mm, and areused as test pieces for plane orientation measurement (C). By effectingthe plane orientation measurement of these two test pieces, the planeorientation of the main body portion is estimated. The positionalrelationship between the respective two test pieces and the ingot bodyis confirmed by the initial reference line of the position. The planeorientation measurement of the test pieces was carried out by the Lauemethod.

[0096] In addition, the entire ingot can be subjected as it is to theplane orientation measurement by the Laue method. However, themeasurement of test pieces yields a greater advantage. This is becausehandling is difficult since the ingot weight reaches as much as severaldozen kilograms, because calcium fluoride has a large coefficient ofexpansion, and its mechanical strength is not large, so that there isthe risk of causing damage to the ingot, and because test pieces(φ30−60×t20) for management of transmittance and excimer laserresistance need to be cut out from the top and cone portions, and thecutting-out step is essential.

[0097] It should be noted since the single crystal of calcium fluoride(or barium fluoride) has a cleaving characteristic at the {111} plane,if the ingot is subjected to a thermal stress or the like, this singlecrystal breaks (cleaves) at the {111} plane. In addition, even in thecase of the ingot which has not cleaved, if its end portion is lightlystruck by a chisel or the like, it cleaves. If, by using this cleavedplane (cleavage plane) as a reference, the ingot is cut so as to beparallel to that plane, it is possible to cut a basic material formanufacturing an optical element. As for the basic material thusobtained, the {111} crystal planes are two parallel planes. Althoughthere is a method in which the cleavage plane is thus used as areference, according to the Laue method, each plane orientation can bemeasured instantaneously on a non-destructive basis.

[0098] A description will be given of an automatic measurement apparatusbased on the Laue method. The automatic measuring apparatus is comprisedof the X-ray source, the sample stage, and the CCD camera.

[0099] The structure provided is such that a sample is flatly placed onthe stage, and the X-ray source and an optical system of the film or CCDcamera are installed therebelow to make it possible to cope with a largesample. The test pieces of the top and cone portions are set such thatthe reference line is located at the front. Since the cone portion isconical, the cone portion is set with its flat portion facing downwardfor measurement. For this reason, measured values of the planeorientation are reversed to collate with the other portions. As theX-ray tube, one using a W target and having a maximum output of 2 kW,e.g., a tube voltage of 50 kV and a tube current of 40 mA, is used TheX-rays generated in the X-ray tube are made incident upon the sampleafter being made substantially parallel by a double pinhole collimatorof 1 mmφ or thereabouts and focused to a beam diameter of 2 mm orthereabouts. The X-ray irradiation time is about 1 minute. Thediffracted beam is projected onto a fluorescent screen, is imaged by theCCD camera, and is fetched into the computer as a Laue pattern. The CCDis cooled to −50° C. by a Peltier element to improve its SN ratio. Thefetched Laue pattern is analyzed on an orientation analyzing screen. TheLaue pattern consists of a plurality of point rows, and one point rowrepresents diffracted spots from the same zone axis of these, if fourpoints are designated by a mouse from the spots (intersections of pointrows) belonging to a plurality of zone axes, indexing is effectedautomatically, and a simulation pattern is displayed by being overlappedwith a Laue pattern when matching is obtained. The degree of agreementof the two patterns is determined by a measurer. If the index isdetermined, a stereographic projection diagram, a stereographictriangle, and plane orientation angles of the respective planes areoutputted as the results of orientation analysis. In a coordinate systemin which the back of the sample stage is x-axis direction, and thevertical downward direction of the sample stage is z-axis direction, theplane orientation angles are expressed by setting as α an angle formedby the z axis and <111>, and by setting as β a angle formed by a lineobtained by projecting <111> onto the measurement plane, in acounterclockwise direction from a+x direction of the x axis. FIG. 4 is aconceptual view of α and β in the cylindrical member.

[0100] Meanwhile, the remaining ingot body with the cone and topportions cut off is subjected to rounding, and the cylindrical surfaceportion is set as a surface equivalent to a sanding finished surface(E). It is also possible to observe an inner portion by surface grindingthe side surface with a width of several centimeters (D). In addition tothe observation from the surface at the sanded surface, the innerportion is observed in a darkroom by applying matching oil for therefractive index, stress concentrations and the like at the interfaceare observed by a cross Nicol optical system, and states of thesub-grain boundary and polycrystals, and the position of the interfaceare confirmed. Furthermore, the conditions of bubbles and foreignobjects are also concurrently confirmed. If the entire ingot has growninto a single crystal, if the plane orientation at one of the cone andtop portions is measured, the plane orientation of the entire ingot canbe estimated. In order to be more accurate, it is preferable to measurethe respective plane orientations at the cone and the top, and confirmthat there is no discrepancy between their plane orientations. Further,there are many cases where the ingot has become polycrystals, andsub-grain boundaries are present. In such a case, it is necessary tomeasure the crystal orientation for each portion of the single crystalin the ingot.

[0101] <Step of Determining Cutting Orientation and Cutting>

[0102] The direction of cutting the ingot is determined on the basis ofα and β determined by the above-described test pieces. When theorientation angle of the top portion is used, a side surface in a(90°−β) direction in a case where when the ingot is viewed from thedirection of the top, the counterclockwise rotation from the directionof the reference line about a plane normal line of a cut plane of thetop portion is set as positive is used as a bearing surface. Meanwhile,a clockwise a direction using the cut section of the top portion as areference plane is set as a cutting direction. When the orientationangle of the cone portion is used, a side surface in a (90°−β) directionin a case where when the ingot is viewed from the direction of the cone,the counterclockwise rotation from the direction of the reference lineabout a plane normal line of a cut plane of the cone portion is set aspositive is used as a bearing surface (reference plane of processing).Meanwhile, a clockwise a direction using the cut section of the topportion as a reference plane is set as a cutting direction (F). Further,a cutting position is determined by incorporating a processing allowancein the annealing step, i.e., dimensions of +5 to 10 mm for boththickness and diameter, into a desired part size (G). In cutting, aground surface (bearing surface) parallel to the axis of the ingot isfirst formed in the bearing surface direction of the ingot side surfaceset from the plane orientation angle, the ingot is placed on the stageof a cutting machine with the bearing surface facing downward. The ingotis cut by being rotated by α by using the top surface as the referenceplane (H). As for the elliptical disk obtained by cutting, its innerportion is observed in a darkroom by applying matching oil for therefractive index thereto, stress concentrations and the like at theinterface are observed by a cross Nicol optical system, and states ofthe sub-grain boundary and polycrystals, and the position of theinterface are confirmed. Furthermore, cutting (rough cutting) (I) androunding (J) are effected after determining the position of the partcutting by incorporating a processing allowance in the annealing step,i.e., dimensions of +5 to 10 mm for both thickness and diameter, into adesired part size while avoiding the positions of bubbles and foreignobjects in the ingot. Further, rough grinding and chamfering areeffected for inspecting the plane orientation (K). In this case, theplane orientation of a final member can also be managed as such amarking that clarifies the relationship with the plane orientationdetermined from the initially marked reference line is maintained alsoin the subsequent steps.

[0103] As the plane-orientation determining step is provided before thecutting-out step, angle of deviation of the manufactured optical memberfrom a desired orientation can be set to within 3°. This angle can beused up to 4°, a maximum of 6° or thereabouts in a most deviating state,but 3° or less is desirable, and 2° or less is particularly desirable.

[0104] The Laue method used in the present invention is not limited tothe management of such plane orientations, but can be also used in thedetection and management of sub-boundaries and twins. Slightsub-boundaries are not easily detected by visual observation, and askilled person normally needs to detect them by obliquely applying thelight to the ground surface. If such mapping measurement is effected, ina case where sub-boundaries are present, deviations of the planeorientation of several degrees or thereabouts are present, they can beeasily detected.

[0105] <Heat Treatment Step>

[0106] As for the two cylindrical basic materials of φ260×t50 andφ200×t60 thus obtained, after their crystal plane (axis) orientations inthe lateral direction are measured, and the amount of birefringence inthat direction and the orientation of the fast axis are measured, heattreatment (annealing) is provided for the improvement of the quality.The measurement of the birefringence of the light transmitted in adirection perpendicular to the normal direction of the two parallelplanes, i.e., the lateral direction of the member, will be referredhereafter to as the measurement in the lateral direction.

[0107] <Step of Measuring Plane Orientation and Birefringence>

[0108] A method similar to the plane orientation measurement of theingot based on the above-described Laue method is used in themeasurement in the lateral direction.

[0109] The plane orientations of the two parallel planes are determinedas being such as the {111} plane and the {100} plane, and themeasurement in the direction of the optical axis is uniform. However, inthe measurement in the lateral direction, arbitrariness of 180° ispresent toward the center of the member. In the case where the twoparallel planes are the {111} planes, the optical axis direction becomesthe <111> axis, but the <110> axis and the <211> axis, for example, arepresent in the lateral direction perpendicular thereto. In the casewhere the two parallel planes are the {100}, the optical axis directionbecomes the <100> axis, but the <100> axis and the <110> axis, forexample, are present in the lateral direction perpendicular thereto. Asa result of making detailed measurement of the lateral direction by thepresent inventors, it was found that certain periodicity is present inthe 180° rotational direction, and that the birefringence in the lateraldirection assumes a maximum (maximal) value in the direction of the<110> axis in the case where the two parallel planes are the {111}planes, and in both the direction of the <110> axis and the direction ofthe <100> axis in the case where the two parallel planes are the {100}planes.

[0110] Accordingly, the relationship between the crystal planeorientation in the lateral direction and birefringence is measured bythe measurement in the lateral direction.

[0111] In this measurement, the crystal plane orientation in a specificlateral direction is measured by, for instance, the Laue method, theamount of birefringence in that orientation is measured, and the amountof birefringence in the lateral direction having a predetermined anglefrom that orientation is consecutively measured. Specifically, in thecase where the two parallel planes are the {111} planes, the amount ofbirefringence in the direction of the <110> axis of the side surface ismeasured. In this case, the <110> axis is present in the 120° rotationaldirection in the lateral direction. Preferably, by using a specific<110> axis direction as a reference, the amount of birefringence ismeasured at angular intervals in the rotational direction, e.g., inunits of 30°, preferably in units of 10° or thereabouts.

[0112] It is true that even if the crystal plane orientation in thelateral direction is not measured by the Laue method, if theabove-described periodicity is made use of, it is possible to estimatethe crystal plane orientation in the lateral direction. Namely, theamount of birefringence is measured in the 180° rotational direction in,for example, units of 10° by using an arbitrary position in the lateraldirection as a reference. The direction of a maximum value thus obtainedbecomes the <110> axis in the case where the two parallel planes are the{111} planes, and the <110> axis or the <100> axis in the case where thetwo parallel planes are the {100} planes.

[0113] As described above, the crystal plane orientation, the amount ofbirefringence, and the orientation of the fast axis in the lateraldirection prior to heat treatment are measured.

[0114] At this time, it is preferable to provide marking to maintain thecrystal orientation of the basic material during annealing. A softpencil or red oil based ink, which does not cause damage to the surfaceof the calcium fluoride and does not produce impurity contamination, isused for the marking. Since red oil based ink turns black afterannealing, the discrimination before and after annealing becomespossible.

[0115] Further, mapping measurement of the in-plane plane orientation ofthe two parallel planes of the basic material is carried out. In theplane orientation measurement involving X-ray irradiation as in the Lauemethod, damage is caused to the calcium fluoride basic material, andcauses a color center. Therefore, the stage of the basic material havingan extra thickness of 2.5 to 5 mm in terms of a final lens shape issuited to mapping measurement. After the shape of the basic material hasbecome closer to the lens shape, only the outside of the effectivediameter in the optical design, i.e., a peripheral range of severalmillimeters, can be undesirably measured.

[0116] In the present invention, it is preferable to precisely managethe angular deviation of the crystal plane. At the time of measuring theangular deviation, an apparatus is used which is based on theside-reflection Laue method for measuring an angular deviation betweenthe sample surface and the crystal plane from the Laue spots obtained byside reflection. As for the Laue method, the back-reflection method orthe transmission method is generally used, in which case it ispreferable to manage the transmittance after X-ray irradiation so as tominimize damage to the sample.

[0117] <Heat Treatment Step>

[0118] Thereafter, the basic material whose crystal plane orientationand birefringence have been measured is subjected to heat treatment soas to improve the optical performance such as birefringence.

[0119] The cylindrical basic material is placed in a container of a heattreatment apparatus so that the flat surfaces become upper and lowersides, and heat treatment (annealing: a heat treatment temperature of1080° C.) is provided by heating by a heater (L).

[0120] The heat treatment apparatus is a vacuum apparatus, and isstructured to prevent the entry of oxygen which causes the haze ofcalcium fluoride. The external structure is made of stainless steel, anda graphite heater and a graphite container are installed in itsinterior. To completely eliminate the internal oxygen and to coat themetal exposed to the in-furnace surface with a fluoride, approximately100 g of acid ammonium fluoride is sealed in the furnace together withcalcium fluoride simultaneously with the calcium fluoride member. Inthat state, after the interior of the furnace is set in a vacuum stateby a vacuum pump, temperature rising is started. Shortly before andafter the in-furnace temperature exceeds 500° C., vaporization of acidammonium fluoride starts, so that the in-furnace pressure turns to aweak positive pressure. The temperature is raised while controlling thepressure so that this weak positive pressure (2 to 8 kPa) will bemaintained, the temperature is then held at 1080° C., and annealing iseffected.

[0121] By effecting the above-described annealing, it is possible toreduce the amount of birefringence attributable to thermal stress inspecific crystal plane orientations of the calcium fluoride member.

[0122] In particular, when the temperature is held at a predeterminedlevel, heat treatment is effected such that the temperature distributioninside the bulk (member) during the temperature drop and radiationalcooling falls within 0.5° C. at both times, thereby making it possibleto reduce the amount of birefringence in predetermined crystal planeorientations of the side surfaces.

[0123] The heat treatment apparatus should preferably be provided with aheat insulating material or a heater disposed in such a manner as tocover the entire surface of the subject material (calcium fluoride basicmaterial). To improve a thermal homogeneity condition in the apparatus,it is preferable to use an apparatus having a sufficiently largecapacity, e.g., a capacity 10-fold or more the member, with respect tothe member.

[0124] In addition, to further improve thermal homogeneity, it ispreferable to rotate the subject material inside the heat treatmentapparatus.

[0125] Alternatively, if a heat treatment apparatus is used in which aheater is disposed to as to bring about such a heat distribution as tooffset the circumferential distribution of the amount of birefringencein accordance with the results of measurement of the crystal planeorientations of the side surfaces of the cylindrical basic material andthe amounts of birefringence at the respective orientations, it ispossible to reduce the amount of birefringence in the lateral directionof the basic material which is the subject material.

[0126] A window is worked in the side surface of the cylindrical basicmaterial subjected to heat treatment, and its upper and lower surfacesare ground by 2.5 mm each (M). Then, the homogeneity of thebirefringence and the refractive index of the side surface are confirmed(N). Subsequently, rounding is effected (O).

[0127] After annealing, polishing (tentative gloss forming) andchamfering are effected (P), and automatic measurement is made of valuesof birefringence of the light traveling in the normal direction of thetwo parallel planes with respect to about 200 points by using anautomatic birefringence measuring apparatus made by ORC ManufacturingCo. Ltd. or Uniopto (measurement wavelength is 633 nm ). Thismeasurement is referred to as the measurement in the optical axisdirection. In addition, measurement is also made of the birefringence ofthe light transmitted in a direction perpendicular to the normaldirection of the two parallel planes, i.e., in the lateral direction ofthe member. In a case where the outer periphery is circular, such anauxiliary tool as to allow the light to straightly travel is necessary,but automatic measurement becomes possible by contriving the holding ofthe member. This measurement is referred to as the measurement in thelateral direction.

[0128] By virtue of the above-described various heat treatment methods,in the present invention, it has become possible to make small anabsolute value of the amount of birefringence in the side surface of thebasic material irrespective of the crystal orientation.

[0129] As the amount of birefringence in the lateral direction is thusreduced, control of intrinsic birefringence attributable to the crystalplane orientation is facilitated. Namely, the birefringence attributableto the thermal stress in the invention is measured and managed at awavelength of 633 nm. However, in order to accurately manage the effectof intrinsic birefringence which becomes large at a wavelength (e.g.,193 nm or the like) which is actually used in an optical member, it isextremely effective to minimize the amount of birefringence attributableto the thermal stress at 633 nm.

[0130] <Example of Projection Exposure Apparatus>

[0131] Next, an example is shown of a projection exposure apparatus onwhich an optical member formed of a fluoride crystal obtained inaccordance with the invention is mounted.

[0132] The projection exposure apparatus shown in FIG. 5 has an F₂ laser(wavelength: 157 nm ) as a light source 11 for supplying illuminatinglight of an ultraviolet zone. The light emitted from the light source 11uniformly illuminates a mask 13 with a predetermined pattern formedthereon through an illuminating optical system 12.

[0133] It should be noted that one or a plurality of bending mirrors forchanging the optical path are disposed, as required, in the optical pathfrom the light source 11 to the illuminating optical system 12. Inaddition, the illuminating optical system 12 is formed by, for example,a fly-eye lens, an internal reflection-type integrator, or the like, andhas an optical system including a field stop for defining a surfacelight source of a predetermined size and shape, as well as afield-stopped image-forming optical system or the like for projecting afield-stopped image onto the mask 13. Further, the optical path betweenthe light source 11 and the illuminating optical system 12 ishermetically sealed by a casing (not shown), and the space from thelight source 11 to the optical member disposed on a side close to themask 13 in the illuminating optical system 12 is substituted by an inertgas (nitrogen, helium, or the like) having a low absorbance of theexposure light.

[0134] The mask 13 is held on a mask stage 15 in parallel to the XYplane by means of a mask holder 14. A pattern to be transferred has beenformed on the mask 13, and, of the entire pattern region, a slit-likepattern region having a long side along the Y-axis direction and a shortside along the X-axis direction is illuminated.

[0135] The mask stage 15 is two-dimensionally movable along the masksurface (XY plane), and its position coordinates are arranged to bemeasured and controlled by an interferometer 17 using a mask movingmirror 16.

[0136] The mask 13, the mask holder 14, and the mask stage 15 thusarranged between the illuminating optical system 12 and a projectionoptical system 18 are accommodated in a casing (not shown), and theinterior of the casing is substituted by an inert gas (nitrogen, helium,or the like).

[0137] The light from the pattern formed on the mask 13 forms a maskpattern image on a wafer 19, i.e., a photosensitive substrate, throughthe catadioptric type projection optical system 18. The wafer 19 is heldon a wafer stage 21 in parallel to the XY plane by means of a waferholder 20. Further, a pattern image is formed on the wafer 19 in aslit-like exposed region having a long side along the Y-axis directionand a short side along the X-axis direction, so as to opticallycorrespond to the slit-like illuminated region on the mask 13.

[0138] The wafer stage 21 is two-dimensionally movable along the wafersurface (XY plane), and its position coordinates are arranged to bemeasured and controlled by an interferometer 23 using a wafer movingmirror 22.

[0139] The wafer 19, the wafer holder 20, and the wafer stage 21 areaccommodated in a casing (not shown), and the interior of the casing issubstituted by an inert gas (nitrogen, helium, or the like).

[0140] Thus, in the projection exposure apparatus shown in FIG. 5, anatmosphere in which the absorption of the exposure light is suppressedis formed in the entire region of the optical path from the light source11 to the wafer 19.

[0141] In addition, the shapes of the illuminated region (field region)of the mask 23 formed by the projection optical system 18 and theprojected region (exposed region) on the wafer 19 are slit shapes havingthe short side along the X-axis direction. Accordingly, as the maskstage 15 and the wafer stage 21, or together with the mask 13 and thewafer 19, are synchronously moved along the short side direction (X-axisdirection) of the slit-like illuminated region and the exposed regionwhile performing position control of the mask 13 and the wafer 19 byusing a drive system, the interferometers 17 and 23, and the like,scanning and exposure are effected on the wafer 19 with respect to theregion having a width equal to the long side of the exposed region and alength corresponding to the amount of scanning (amount of movement) ofthe wafer 19.

[0142] Then, as optical members (lenses, prisms, and the like)constituting the illuminating optical system 12 and the projectionoptical system 18, it is effective to use optical members which havebeen subjected to management of two or more crystal plane orientationsin accordance with the invention.

[0143] The projection exposure apparatus shown in FIG. 5 is one example,and optical members fabricated in accordance with the invention may beapplied to various projection exposure apparatuses such as thosedisclosed in U.S. Pat. No. 6,341,007B1.

[0144] <Example of Projection Optical System>

[0145]FIG. 6 is a schematic view illustrating one example of theprojection optical system used in the projection exposure apparatus inaccordance with the invention.

[0146] In FIG. 6, the projection optical system has a firstimage-forming optical system G1 of a catadioptric type for forming anintermediate image of the pattern on a reticle R serving as a projectionoriginal plate, as well as a second image-forming optical system G2 of arefraction type for allowing the intermediate image obtained by thefirst image-forming optical system G1 to be formed again on a wafer Wserving as a workpiece. Disposed on an optical axis AX1 is anoptical-path bending member having an optical-path-bending reflectingmirror 31 having a reflecting surface S1 for deflecting the optical path90° from the reticle R toward the first image-forming optical system G1and a reflecting surface S2 for deflecting the optical path 90° from thefirst image-forming optical system G1 toward the second image-formingoptical system G2.

[0147] The first image-forming optical system G1 has a plurality of lenscomponents and a concave reflecting mirror arranged along the opticalpath AX1, and forms an intermediate image with a substantially equalmagnification or a slightly reduced magnification.

[0148] The second image-forming optical system G2 has a plurality oflens components a arranged on an optical path AX2 perpendicular to theoptical axis AX1 as well as a variable aperture stop AS for controllingthe coherence factor, and forms a secondary image with a predeterminedreduced magnification on the basis of the light from the intermediateimage.

[0149] Here, an optical axis AX0 in FIG. 6 is an optical axis which islocated between the reticle R and a reflecting mirror 31 and which isperpendicular to the optical axis AX1 of the first image-forming opticalsystem G1. The optical axis AX0 and the optical axis AX2 may align on anidentical straight line.

[0150] In addition, FIG. 6 shows the projection optical system providedwith the first image-forming optical system G1 and the secondimage-forming optical system G2 respectively having a plurality of lenscomponents, but the lens components arranged along the optical axes AX1and AX2 may be either singular or plural.

[0151] Furthermore, the angle formed by the optical axis AX0 and theoptical axis AX1 may not necessarily be 90°, and may be an angleobtained by, for example, rotating a concave reflecting mirror CMcounterclockwise. At this time, it is preferable to set the angle ofbending the optical axis by a reflecting surface S2 such that thereticle R and the wafer W become parallel.

[0152] In addition, in the present invention, it is also possible to usea projection optical system having two reflecting mirrors 31 and 32, asshown in FIG. 7.

[0153] Further, in the present invention, it is also possible to use aprojection optical system having the configuration shown in FIG. 8.

[0154] In FIG. 8, the projection optical system has the firstimage-forming optical system G1 of the catadioptric type for forming anintermediate image of the pattern on the reticle R serving as aprojection original plate. A first optical-path-bending reflectingmirror 31 is disposed in the vicinity of a first intermediate imagewhich is formed by the first illuminating optical system G1. The lightbeam directed toward the first intermediate image or the light beam fromthe first intermediate image is deflected toward the second illuminatingoptical system G2 by the first optical-path-bending reflecting mirror31. The second illuminating optical system G2 has the concave reflectingmirror CM and at least one negative lens 33, and forms a secondintermediate image (an image of the first intermediate image and asecondary image of the pattern) of a substantial equal magnification tothat of the first intermediate image on the basis of the light beam fromthe first intermediate image.

[0155] A second optical-path-bending reflecting mirror 32 is disposed inthe vicinity of the position for forming the second intermediate imagewhich is formed by the second illuminating optical system G2. The lightbeam directed toward the second intermediate image or the light beamfrom the second intermediate image is deflected toward a thirdilluminating optical system G3 by the second optical-path-bendingreflecting mirror 32. It should be noted that the reflecting surface ofthe first optical-path-bending reflecting mirror 31 and the reflectingsurface of the second optical-path-bending reflecting mirror 32 arearranged so as not to spatially overlap with each other.

[0156] The third illuminating optical system G3 forms a reduced image(an image of the second intermediate image and a final image of thecatadioptric optical system) of the pattern of the reticle R on thewafer W serving as a workpiece (a photosensitive substrate) disposed ona second surface on the basis of the light beam from the secondilluminating optical system.

[0157] The projection optical systems shown in FIGS. 6 to 8 are suitablyused in cases where, for instance, the exposure light source is an F₂laser. Meanwhile, in a case where the exposure light source is an ArFexcimer laser, a projection optical system having, for instance, a lensconfiguration shown in FIG. 9 is suitably used.

[0158] In FIG. 9, a first lens group G1 of positive power, a second lensgroup G2 of positive power, and a third lens group G3 of negative powerare formed in that order from the side of the reticle R serving as afirst object. This projection optical system is substantiallytelecentric on the object side (reticle R side) and the image side(wafer W side), and has a reduced magnification. Further, the numericalaperture (N.A.) of this projection optical system is 0.6, itsmagnification of projection is ¼, and the diameter of the exposed regionon the image side is 30.6.

[0159] In a case where the projection optical system has a lensconfiguration shown in FIG. 9, the material of each lens is normallyselected appropriately to correct chromatic aberrations. For example,correction of chromatic aberrations can be suitably effected by usingsilica glass as the material of 14 lenses L11 to L114 constituting thefirst lens group G1, silica glass as the material of 4 lenses L21 to L24constituting the second lens group G2, a calcium fluoride crystal as thematerial of the lenses L31, L33, L35, L37, L38, and L310 among 11 lensesL31 to L311, and silica glass as the material of the remaining 5 lenses,respectively constituting the third lens group G3.

EXAMPLE 1

[0160] An ingot was fabricated by using the Bridgman method. Ahigh-purity material made by chemical synthesis was used as a rawmaterial. Graphite crucibles with the powdered material filled thereinwere placed in a superposed manner inside in a growing apparatus, andthe interior of the growing apparatus was kept in a vacuum atmosphere of10⁻³ to 10⁻⁴ Pa. Next, the temperature within the growing apparatus wasraised to a level above the melting point of calcium fluoride to meltthe raw material, and the temperature was subsequently lowered to roomtemperature. In this case, PID control was carried out to suppress thetemporal change of the temperature within the growing apparatus. Leadfluoride as a fluorinating agent was added to this powdered material. Asemi-molten article thus obtained was moved to a crystal growingfurnace, and after the temperature was raised again to the meltingtemperature, the crucible was lowered at a speed of 0.1 to 5 mm/h orthereabouts in the crystal growing stage, thereby allowingcrystallization to gradually take place starting from a lower portion ofthe crucible. When crystallization occurred up to an uppermost portionof the melt, crystal growth was completed, and annealing was effected byavoiding quenching so that the grown crystal (ingot) would not fracture.When the temperature within the growing apparatus lowered to roomtemperature or thereabouts, the apparatus was opened to atmosphere, andan ingot of φ290×t300 mm was removed.

[0161] In this crystal growth, a pencil-type crucible whose tip portionwas conical was manufactured by using a φ300-mm graphite crucible. Aseed crystal was inserted in a tip portion of the conical portionlocated at a lower end of the crucible to allow a crystal to grow whilethe plane orientation in crystal growth was being controlled, therebyobtaining a single crystal. Since the ingot which is removed has a verylarge residual stress, the temperature of the ingot in the furnace wasallowed to drop gradually down to room temperature.

[0162] The conical shape (referred to as the cone portion) at the tip ofthe ingot of the calcium fluoride single crystal thus obtained, as wellas its opposite portion (referred to as the top portion), wererespectively cut to a thickness of 30 mm (diameter: 290 mm ), and thepieces thus cut out were used as test pieces for measuring of planeorientation. The plane orientation of the main body portion wasestimated by measuring the plane orientation of these two test pieces.The measurement of the plane orientation of the test pieces was effectedin accordance with the Laue method.

[0163] The plane orientations of two cylindrical basic materials ofφ260×t50 and φ200×t60, in which the plane orientations of the twoparallel planes were {111} planes, were cut out, and birefringence wasmeasured. As for objects to be measured, measurement was made of thein-plane distribution of birefringence of the {111} plane, as well as 18directions in units of 10° by using the <110> direction as a referencein accordance with the crystal plane orientation of the side surfacedetermined in advance by the Laue method.

[0164] As a result of measurement, the birefringence in the optical axisdirection at the {111} crystal plane before heat treatment was 10 nm/cmor more. In addition, as for the lateral direction, the <110> directionexhibited a maximum value of 12 nm/cm. Accordingly, heat treatment ofthis basic material was carried out.

[0165] The cylindrical basic material was placed in a heat treatmentapparatus so that the flat surfaces became upper and lower sides, andheat treatment (heated to and held at 1080° C., followed by annealing)was provided by heating by a heater. At that time, adjustment of thetemperature schedule was made so that the temperature distributioninside the member fell within 0.5° C. during each time of holding,temperature drop, and radiational cooling.

[0166] The heat treatment apparatus was a vacuum apparatus, and wasstructured to prevent the entry of oxygen which causes the haze ofcalcium fluoride. The external structure was made of stainless steel,and a graphite heater and a graphite container were installed in itsinterior.

[0167] As for the graphite container, one having a sufficiently largecapacity (a capacity 10-fold or more that of the basic material) withrespect to the member was used. Approximately 100 g of acid ammoniumfluoride was sealed in the graphite container together with the calciumfluoride member. After the interior of the furnace was set in a vacuumstate by a vacuum pump, temperature rising was started. The temperaturewas raised while controlling the pressure so that the in-furnacetemperature was kept at a weak positive pressure (2 to 8 kPa), thetemperature was then held at 1080° C., and annealing was effected.

[0168] As a result of the above-described heat treatment, thebirefringence in the optical axis direction at the {111} crystal planeafter the heat treatment of the calcium fluoride member became 0.8nm/cm. In addition, as for the lateral direction, the <110> directionexhibited a maximum value of 2.5 nm/cm. The maximum value of thisbirefringence satisfies the requirement as a lens optical member of aprojection optical system used for an exposure system. For this reason,a lens optical member was formed by using this basic material.

EXAMPLE 2

[0169] A calcium fluoride single-crystal ingot was grown by a methodsimilar to that of Example 1. A plurality of cylindrical members werecut out from this ingot such that the {100} planes became the two upperand lower planes. The birefringence in the optical axis direction at the{100} crystal planes of the cut-out members was 20 nm/cm or more (beforeheat treatment). In addition, the birefringence in the lateral directionof this member exhibited a maximum value of 18 nm/cm in the <110>direction. Accordingly, heat treatment was carried out in the same wayas in Example 1. As a result of this heat treatment, the birefringencein the optical axis direction at the {100} crystal planes of the calciumfluoride member became 2.8 nm/cm. In addition, the birefringence in thelateral direction exhibited a maximum value of 5.9 nm/cm in the <110>direction. The maximum value of this birefringence satisfies therequirement as a lens optical member of a projection optical system usedfor an exposure system. For this reason, a lens optical member wasformed by using this basic material.

EXAMPLE 3

[0170] A calcium fluoride single-crystal ingot was grown by a methodsimilar to that of Example 1. A plurality of cylindrical members werecut out from this ingot such that the {100} planes became the two upperand lower planes. The birefringence in the optical axis direction at the{100} crystal planes of the cut-out members was 20 nm/cm or more (beforeheat treatment). In addition, the birefringence in the lateral directionexhibited a maximum value of 18 nm/cm in the <110> direction.Accordingly, heat treatment of this member was carried out.

[0171] The cylindrical basic material was placed in the heat treatmentapparatus so that the flat surfaces became upper and lower sides, andheat treatment (heated to and held at 1080° C., followed by annealing)was provided by heating by a heater. The heat treatment apparatus was avacuum apparatus, and was structured to prevent the entry of oxygenwhich causes the haze of calcium fluoride. Further, the heat treatmentapparatus had a piping structure capable of introducing a gas-basedfluorinating agent and was made of a material which was highly corrosionresistant. Specifically, the external structure was made of stainlesssteel, and a graphite heater and a graphite container were installed inits interior. The calcium fluoride was placed in this graphitecontainer, and there was a mechanism for rotating this graphitecontainer about the center of the member, and this graphite containerwas rotated in the range of 1 to 5 rpm.

[0172] As for the graphite container, one having a sufficiently largecapacity (a capacity 10-fold or more that of the basic material) withrespect to the member was used. Approximately 100 g of acid ammoniumfluoride was sealed in the graphite container together with the calciumfluoride member. After the interior of the furnace was set in a vacuumstate by the vacuum pump, temperature rising was started. Thetemperature was raised while controlling the pressure so that thein-furnace temperature was kept at a weak positive pressure (2 to 8kPa), the temperature was then held at 1080° C., and annealing waseffected. The heat treatment conditions were set similar to those ofExample 1. As a result of this heat treatment, the birefringence in theoptical axis direction at the {100} crystal plane of the calciumfluoride member became 2.2 nm/cm. In addition, the birefringence in thelateral direction exhibited a maximum value of 3.4 nm/cm in the <100>direction.

EXAMPLE 4

[0173] A calcium fluoride single-crystal ingot was grown by a methodsimilar to that of Example 1. A plurality of cylindrical members werecut out from this ingot such that the {111} planes became the two upperand lower planes. The birefringence in the {111} crystal planeorientation of the cut-out members was 10 nm/cm or more (before heattreatment). In addition, the birefringence in the lateral directionexhibited a maximum value of 11 nm/cm in the <110> direction.Accordingly, heat treatment was effected at a faster cooling rate thanthat of the condition in Example 1. As a result of this annealing, thebirefringence in the {111} crystal plane orientation after heattreatment of the calcium fluoride member became 1.5 nm/cm. Meanwhile,the birefringence in the lateral direction was 5.9 nm/cm in measurementin an arbitrary direction. However, in the measurement in the <110>direction, the birefringence exhibited a maximum value of 12 nm/cm.Accordingly, since this maximum birefringence value is unsuitable(nonstandard) as an optical member of a lens for a projection opticalsystem of the exposure apparatus, this member was not used as a materialfor lens forming. In addition, as a result of this example, it wasrevealed that even if the birefringence in the lateral direction ismeasured in an arbitrary direction (angle), the material cannot beevaluated sufficiently.

EXAMPLE 5

[0174] A calcium fluoride single-crystal ingot was grown by a methodsimilar to that of Example 1. A plurality of cylindrical members werecut out from this ingot such that the {100} planes became the two upperand lower planes. The birefringence in the {100} crystal planeorientation of the cut-out members was 20 nm/cm or more (before heattreatment). In addition, the birefringence in the lateral directionexhibited a maximum value of 18 nm/cm in the <110> direction.Accordingly, heat treatment was effected at a faster cooling rate thanthat of the condition in Example 2. As a result of this heat treatment,the birefringence in the {100} crystal plane orientation of the calciumfluoride member became 3.8 nm/cm. Meanwhile, the birefringence in thelateral direction was 7.5 nm/cm in measurement in an arbitrarydirection. However, when measurement was made with respect to the <110>direction, the birefringence exhibited a maximum value of 15 nm/cm.Accordingly, since this maximum birefringence value is unsuitable(nonstandard) as an optical member of a lens for a projection opticalsystem of the exposure apparatus, this member was not used as a materialfor lens forming. In addition, in the same way as Example 4, it wasrevealed that even if measurement is made in the lateral direction in anarbitrary direction, the material cannot be evaluated sufficiently.

[0175] In the present invention, the in-plane (side surface) crystalorientation perpendicular to the optical axis is determined in advance,and the amount of birefringence in a particular crystal planeorientation is measured and managed, thereby making it possible tominimize the effect of the birefringence attributable to the thermalstress on the performance of the optical system.

[0176] In accordance with the present invention, it is possible toprovide an optical member whose crystal plane orientation other than theoptical axis direction is managed, and it becomes possible to ensuredesired optical performance. It is possible to provide an exposureapparatus having high resolution by configuring a projection opticalsystem of a projection exposure apparatus employing, as a light source,ultraviolet rays of such as an excimer laser and an F₂ laser by usingthe optical member thus obtained.

What is claimed is:
 1. A method for manufacturing an optical member of afluoride crystal, comprising: a growing step of growing an ingot of afluoride crystal; a cutting-out step of cutting out from the ingot acylindrical basic material with two parallel planes which have a certaincrystal plane orientation; an orientation-determining step ofdetermining a crystal orientation of a side surface of the cylindricalbasic material; a birefringence-measuring step of measuringbirefringence in a specific crystal axis direction at the side surfacedetermined based on the crystal orientation determined in theorientation-determining step; and an evaluating step of evaluating thefluoride crystal on the basis of a result of measurement of thebirefringence.
 2. The method for manufacturing an optical member of afluoride crystal according to claim 1, wherein, in theorientation-determining step, the crystal orientation at a side surfaceis determined by measuring the birefringence at the side surface at aplurality of angles.
 3. The method for manufacturing an optical memberof a fluoride crystal according to claim 1, wherein, in the evaulatingstep, a determination is made as to whether or not a maximum value ofthe birefringence in the specific crystal axis direction at the sidesurface is not more than 10 nm/cm at a measurement wavelength of 633 nm.4. The method for manufacturing an optical member of a fluoride crystalaccording to claim 3, wherein when the maximum value of thebirefringence in the specific crystal axis direction at the side surfaceis not more than 10 nm/cm at the measurement wavelength of 633 nm, thebasic material is formed into a shape of a predetermined optical member.5. The method for manufacturing an optical member of a fluoride crystalaccording to claim 3, wherein the two parallel planes are {111} planes,and the specific crystal direction at the side surface is <110>direction.
 6. The method for manufacturing an optical member of afluoride crystal according to claim 3, wherein the two parallel planesare {100} plane, respectively, and the specific crystal direction at theside surface is <100> direction or <110> direction.
 7. An optical memberof a fluoride crystal manufactured by the manufacturing method definedin claim 1, wherein a maximum value of the birefringence in a specificcrystal axis direction at a side surface of the fluoride crystal, whichis shaped in a cylindrical shape with two parallel planes having aspecific crystal plane orientation, is not more than 10 nm/cm at ameasurement wavelength of 633 nm.
 8. The optical member of a fluoridecrystal according to claim 7, wherein the specific crystal planeorientation is {111}, and the specific crystal axis direction at theside surface is <110> direction.
 9. The optical member of a fluoridecrystal according to claim 7, wherein the specific crystal planeorientation is {100}, and the specific crystal axis direction at theside surface is <100> direction or <110> direction.
 10. The opticalmember of a fluoride crystal according to claim 7, wherein the fluoridecrystal is a calcium fluoride single crystal.
 11. An exposure apparatusincluding an optical system comprised of the optical member defined inclaim 7.